1. FIELD OF THE INVENTION
This invention relates to a process for producing lower alcohol esters of fatty acids. More particularly, the present invention relates to an improved process for producing lower alcohol esters of fatty acids from unrefined fats which contain impurities, such as polypeptides and phospholipids in addition to free fatty acids, by the interesterification method.
2. DESCRIPTION OF THE PRIOR ART
Lower alcohol esters of fatty acids have many applications. For example, they are used as a raw material for the production of higher alcohols by means of catalytic hydrogenation, and various surface active agents. They are generally produced either by interesterification between fats and lower alcohols or by esterification in which the fatty acids obtained by hydrolysis of the fats are esterified with a lower alcohol. On the industrial scale, however, the interesterification method has generally been employed between fats and lower alcohols in the presence of an alkali catalyst.
It is usual that unrefined fats contain about 2 to 5% of free fatty acids and impurities such as polypeptides, and phospholipids, although this depends on the kind of fat that is used. If such unrefined fats are used as a raw material for the interesterification reaction as they are, the alkali catalyst is consumed also by the free fatty acids contained therein. Unless a larger amount of the alkali is used, the interesterification reaction does not proceed with ease. Furthermore, due to the emulsifying and solubilizing action of the resulting soap of the fatty acids, a greater amount of the fats are dissolved in the glycerol layer during the interesterification. It causes such problems as deterioration of the qualities of the ester and glycerol, and increase in the loss of the materials, and so forth.
To solve these problems, various methods have been applied to the interesterification reaction between fats and lower alcohols using an alkali catalyst at a relatively low temperature not higher than 100.degree. C. For example, a preparative refining step is incorporated before the interesterification step to remove impurities such as free fatty acids from the reaction system, as illustrated in the following methods (1) through (3), and other specific treating steps are incorporated, as illustrated in the following methods (4) and (5).
(1) Alkali refining method:
An alkali such as caustic soda is added to change the free fatty acids to fatty acid soaps, and these are removed by washing. (described, for example, in Yushi Kagaku (Fat Chemistry)", Iwanami Zensho).
(2) Solvent extraction method:
Utilizing the solubility difference in a solvent (e.g. methanol) between the fatty acid and the triglyceride, the free fatty acid is transferred into the solvent phase and is extracted for removal. (U.S. Pat. Nos. 1,371,342, 2,345,097, etc.)
(3) Steam refining method:
Steam is blown in under vacuum to distill off the free fatty acid together with the steam. (Refer to "Yushi Kagaku Kogyo (Fat Chemical Industry), Kogyo Gijutsu Shinsho)".
(4) Excess addition method:
In addition to the catalytic amount sufficient for the interesterification, an alkali is added in an excess amount corresponding to the free fatty acid.
(5) Pre-esterification method:
The free fatty acid is first esterified using an acid catalyst and the interesterification of the fat is then conducted using an alkali catalyst. (Japanese Patent Publication No. 1,823/1960, Japanese Patent Laid-Open Publication No. 62,926,/1975, etc.) Among the conventional methods as mentioned above, the method (1) makes it possible to efficiently remove not only the free fatty acid, but also other impurities. However, this method requires after-treatment of the removed soap, only a low yield of the product is obtained and the cost is increased. In order to sufficiently lower the acid value (AV) and to remove free fatty acids, the method (2) requires a considerably large amount of a solvent. When methanol is used as the solvent in order to reduce the AV from about 10 down to 0.8, for example, methanol must be used in an amount of five times as much by weight as coconut oil. In addition, method (2) cannot substantially remove impurities such as polypeptides and phospholipids.
The method (3) requires relatively large equipment and involves the problem of treating the distillates. As mentioned above, the method (4) involves the problems of deterioration of the quality of the product and increase of the loss of the materials due to the emulsifying and solubilizing action of the resulting fatty acid soap. On the other hand, the method (5) can be relatively easily used on the industrial scale. It has so far been reported, however, that impurities such as polypeptides and phospholipids are hardly removed and so they remain in the fat, even though free fatty acids could be esterified. If such fat is subjected to the interesterification reaction, it is difficult to recover and refine glycerol which contains almost all the impurities, whereby the loss of glycerol increases, because the impurities are almost entirely distributed into the glycerol layer. Moreover, those impurities that remain in the oil will lower the catalytic hydrogenation reactivity of the resulting lower alcohol ester of the fatty acid.